Fine Gael has warned the Garda's witness protection programme must be put on a statutory basis to avoid serious criminals escaping conviction.
Fine Gael Justice spokesman Jim O'Keeffe
Justice spokesman Jim O'Keeffe said convictions could be jeopardised unless the scheme is overhauled.
The Witness Protection Scheme was launched in 1997 as part of the fight against organised crime. The evidence of protected witness Russell Warren was central in the conviction of Brian Meehan for the murder of journalist Veronica Guerin. Meehan is currently contesting his conviction.
Testimony from Warren and two other protected witnesses - Charles Bowden and John Dunne - helped convict John Gilligan of drug dealing.
The scheme was also used by gardai in July 2005 in helping secure the conviction of two men for murdering a 25-year-old man outside Cloverhill Prison in 2004.
In 2003, the State spent €1,183,482 on housing and supporting witnesses and their families. Last year, the cost dropped to €540,547.
The Garda has consistently refused to disclose how many people are under protection at any one time, citing security reasons.
Mr O'Keeffe said today the figures showed the scheme was not being used effectively and argued this is because it was never properly thought out.
"There is no provision in law for an explicit agreement that an accomplice giving evidence against major criminals will not be prosecuted," he said, adding that this fact is deterring criminals from giving evidence.
Unless Minister for Justice Michael McDowell confronts this issue, he said, "it may jeopardise future trials and hamper the State's efforts to get serious criminals behind bars".
He called for a senior judge to be appointed to conduct a full review of the scheme. The last review, which was carried out by Garda Headquarters, was not made public.